In prior art polymers consisting of repeating thiophene units have been reported to show good performance as charge transporting materials in FET applications. For example, regioregular poly(3-alkyl)thiophene for example has demonstrated one of the highest recorded mobilities to date for a polymer (Sirringhaus et al., Science, 1998, 280, p 1741). Also, polythiophene analogues as disclosed for example in EP 1 327 646 A1, EP 1 327 647 A1, EP 1 329 474 A1 or EP 1 329 475 A1 containing different numbers and regioisomers of alkyl-thiophenes exhibit reasonable charge carrier mobilities.
This performance is thought to be due to two factors. Firstly the arrangement of the alkyl side-chains on the polymer backbone allows the polymers to self-organise into well-ordered structures on coating from solution. This facilitates the hopping mechanisms that dominate charge transport. Secondly the presence of sulfur atoms in the polymer backbone has been shown to be beneficial to charge transport. The exact mechanism is not known, but it is speculated that interaction of the sulfur d-orbitals on adjacent polymer chains facilitates the charge hopping mechanism.
However, the polymers disclosed in the above prior art documents do only show charge carrier mobilities of not more than 0.1 cm2V−1s−1. Also, the materials of prior art often show only limited solubility which is a disadvantage when processing the polymers for the manufacture of semiconductor devices like thin film transistors (TFT) or field effect transistors (FET).
Therefore, further enhancement of the charge mobility and solubility of organic polymers is desired in order to enable transistor performance.
It is an aim of the present invention to provide new organic materials for use as semiconductors or charge transport materials, which are easy to synthesise, have high charge mobility and good processibility. Especially the materials should be easily processible to form thin and large-area films for use in semiconductor devices. Also, the materials should be oxidatively stable, but retain or even improve the desired electrical properties.
The inventors of the present invention have found that materials based on thieno[3,2-b]thiophene (1) (TT)
in particular oligomers and (co)polymers of TT comprising 3,6-disubstituted TT groups and/or substituted thiophene or selenophene groups, show improved charge carrier mobility whilst maintaining desirable solution processable properties.
TT materials are known in prior art. For example, Nakayama et al., Heterocycles 1994, 38, p. 143 report 3,6-dimethyl TT. Saidman et al., J. Appl. Electrochemistry, 2001, 31, p 839 report the electropolymerisation of 3,6-dimethyl TT which, however, only gives an insoluble polymer. Nakayama et al., Tetrahedron 1996, 52, p. 471 report dimers, trimers and tetramers of 3,6-dimethyl TT. Again the tetramers are of very low solubility and no electrical properties are reported.
Fuller et al., J. Chem. Soc. Perkin Trans. 1997, 1, 3465-70 report 3,6-disubstituted TT with thioalkyl (—S-Me, —S-Ph) and silylalkyl (—Si(Me)3) substituents. However, polymers are not disclosed. Also, thioethers are often undesired because the electron rich nature of these side chains affords polymers that are oxidatively unstable.
In prior art there are also some reports of polymers containing unsubstituted TT. Kossmehl et al., Makromol. Chem. 1982, 183, p. 2747 report co-polymers of TT with e.g. a vinylene linker group. However, these polymers were poorly soluble and their electrical characterisation was not reported. Rutherford et al., Macromolecules, 1992, 125, p. 2294 reports the synthesis of TT with alkynyl linkers and of electropolymerised unsubstituted or monomethylated TT. Again all such polymers were insoluble.
WO 99/12989 discloses oligomers and polymers comprising two or more fused thiophene rings which may be substituted or unsubstituted for use in TFTs and FETs. However, there is no specific disclosure of (co)polymers of disubstituted TT or their preparation.
Thus, another aim of the invention is to provide thieno[3,2-b]thiophene (TT) materials that are more easily processible in the manufacture of semiconductor devices, have higher stability and allow easier synthesis also at large scale compared to TT materials of prior art.
It was found that the above aims can be achieved by providing monomers, oligomers and (co)polymers according to the present invention.